Converting iron salt solutions into iron oxides by high temperature hydrolysis is well known in the art. In many of the usual embodiments as disclosed in U.S. Pat. No. 379,872, U.S. Pat. No. 2,155,119 and British Patent No. 793,700, the sole object of the invention is simply to produce an oxide particle without regard to controlling either size, shape or crystal phase. In these processes, cost is minimized by introducing large salt droplets into relatively low temperature gases. Since the particle size of the oxides is dependent upon droplet size, the iron oxide particles produced are also large. The large particle size, however, adversely effects the magnetic properties, specifically coercivity.
In order to overcome these limitations, several attempts have been made to use high temperature reactors to produce fine magnetic oxide particles. At least three publications, "Sintering of Ferrite Powder Prepared by a New Spray-Roasting Technique", Sintering and Related Phenomena--Proceedings of International Conference, June 1965, pp. 947-58; "Preparation of Ferrites by the Atomizing Burner Technique", Conference on Magnetism and Magnetic Materials, (IEEE) Publication T-91 (1957) pp. 526-530; and "Fine-Grained Ferrites, I. Nickel Ferrite", Journal of Applied Physics, Supplement to Volume 32, No. 3, March 1961, pp. 237s-238s, teach the formation of metal oxides by decomposing metal precursor solutions in a flame environment. Although oxide particles are indeed produced by these thermal decomposition processes, the particles which form are not single, equiaxed solids but, rather, aggregates of hollow cenospheres which lack attractive magnetic properties. The morphological characteristics of these oxides clearly indicate that the feed solutions utilized were introduced into the reactor at too low a temperature and that the droplets utilized were much too large.
A method of converting nickel chloride or zinc chloride solutions into nickel ferrite or zinc ferrite particles by decomposing metal salt droplets in a counter-current gas stream was taught in U.S. Pat. No. 3,378,335. This process is limited, however, since it will only produce magnetic phases if the mole ratio of nickel or zinc to iron is one to two. Moreover, the average droplet size of the feed solution was large, ranging between 50 and 200 microns, such that the low temperature decomposition of the large droplets again failed to produce fine, unaggregated, single phase oxide particles.
The manufacture of extremely fine magnetic ferric oxide powders by hydrolyzing iron chloride at elevated temperatures was taught in U.S. Pat. No. 2,950,955 and Japanese Patent Application No. 56-149330. The iron chloride introduced into the reactor is in a vapor state rather than a liquid state and is formed by either sublimating an anhydrous material or by contacting a chlorine gas with an iron element. Needless to say, by vaporizing the iron chloride prior to its introduction into the reactor, the process of forming fine particle oxides becomes inherently complex and expensive.
In a publication entitled "Ultrafine Metal Oxides by Decomposition of Salts in a Flame", Proceeding of the Electrochemical Society Symposium--Ultrafine Particles, May 3, 1961, pp. 181-195, the authors describe a method for making magnetic oxide powders from ferrous chloride solutions. Ferrous chloride is first converted to ferrous oxide which subsequently oxidizes to magnetite. The magnetite in turn oxidizes to gamma ferric oxide. Since gamma ferric oxide is metastable and will convert to alpha ferric oxide at temperatures above 550.degree. C., the hot gamma ferric oxide particles are cooled before the gamma ferric oxide can convert to the equilibrium phase. The undesirable feature of this process, obviously, is that the particles may reach temperatures above 550.degree. C. for only very short time periods.